Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.
Material impurities and defects in the organic layers increase non-radiative decay mechanisms and increase material reabsorption of photoemissions, resulting in an increase in heat generation, a lowering of the internal quantum efficiency, and a decrease in the life of the device.
A mechanism which produces defects during layer deposition is thermal decomposition. Thermal decomposition occurs when thermal energy causes bonds within organic molecules or chains to disassociate, breaking the organic material down into constituent components. The constituent components may also produce outgassing and secondary reactions. Thermal decomposition also alters the vaporization rate of the organic material used to form the organic layer.
Methods have been developed to reduce thermal decomposition and to improve the controllability of the vaporization rate. For example, U.S. Pat. No. 6,797,337 to Dando et al. (“Dando”) discloses a method for reactively growing metallic layers by evaporating a metal organic precursor.
In Dando, a solid precursor comprising an organic metal-containing compound is vaporized by exposing a large area of the surface of the precursor to infrared radiation, sublimating the precursor at the surface without the necessity of heating the entire volume. A non-reactive carrier gas then transports the vaporized precursor to a reaction chamber containing a substrate. In the reaction chamber, the vaporized precursor undergoes a chemical reaction when mixed with separately injected chemical species, whereupon the desired metal is deposited on the substrate.
Many solid precursors decompose over time when held near their sublimation temperatures, especially if the decomposition temperature is below the sublimation temperature. By minimizing the heating of the bulk, Dando seeks to minimize bulk decomposition and thereby gain better control over the vaporization rate.
To sublimate a solid it is necessary to impart sufficient energy to transition the solid directly into vapor without melting. In the system of Dando, this can require a relatively large amount of energy to overcome the large temperature gradient between the bulk and the surface, which may result in decomposition of the organic material in the vapor. Vapor decomposition is of little consequence, since the precursor in Dando is purposely unstable, undergoing a chemical reaction prior to deposition and having some constituent components discarded in the reactor.
While the system of Dando does have some advantages for precursor-based deposition of films, it may not be well suited to precursor-free systems. Precursor-free systems vaporize a solid organic material, having the vapor condense on a substrate to assemble into a layer of solid organic material having the same molecular composition as the original target material. The absence of precursor components eliminates a source of contamination in the assembled organic layer. However, decomposition of either the bulk or the vapor diminishes the quality of the assembled organic layer and can compromise the operational efficiency of a finished device.
Another problem that may occur in the system of Dando is material instability produced by thermal cycling. While it is generally efficient to use a same target to create multiple layers, each time heating is suspended, residual energy from the surface dissipates into the bulk. These thermal oscillations can create molecular instability in solid organic materials sensitive to thermal history. Even when this instability does not create decomposition in the bulk, it can increase the likelihood of decomposition in the vapor.
An example of another method to reduce thermal decomposition and to improve the controllability of the vaporization rate is disclosed in U.S. Pat. No. 5,094,880 to Hongoh (“Hongoh”). Hongoh discloses a precursor-free system for the deposition of organic superconductor materials. Organic superconducting materials are particularly unstable, in comparison to inorganic semiconductors and insulators. Organic superconducting materials have relatively low sublimation temperatures and heat resistance and are prone to decomposing immediately upon vaporization by ordinary methods. Hongoh irradiates an actively cooled target with focused light energy to instantaneously heat a small surface portion of the target to sublimation. The energy density of the light is restrained to minimize decomposition of the material. Breakdowns in molecular structure are further reduced by applying pulsed light to shorten the heating time.
While the method of Hongoh offers several improvements over Dando, the temperature gradient at the surface of the cooled target in combination with the restraints on energy density results in a very slow deposition process. For the mass-manufacturing of organic components such as OLEDs, such a slow process is commercially impractical.